Honestly, the nicer buildings are the newer ones, and they tend to be further out of town than the older, mature estates. I say tend to be, there is one obvious exception, The Pinnacle - a design award-winning HDB that has just passed its 5th birthday. (This is significant because there is a Minimum Occupancy Period of 5 yrs on all HDBs when purchased, so its only in the last couple of months that apartments at The Pinnacle can be legally rented out as a whole unit). It's also slap bang on the edge of the CBD, so expect
condo-like prices for rentals (the sales prices are >$1m).
But don't confuse an older building for an older HDB apartment, some are renovated to "condo" standards inside. And as they are built by the government, as SMS says, all resources are made available. Far more so than most
condos.
One more thing - older units tend to be larger before space got so scarce. I've still never seen a condo - even the expensive ones on Sentosa - with a kitchen or bedrooms as big as my current 3-room (2bed) HDB. I can fit three king-size beds in one of my bedrooms!